A large body of knowledge concerning the bonding of aluminum and titanium metals has been developed largely by the aerospace industry. Relatively little information is available regarding the bonding of steels, particularly with use of the most important class of adhesives, viz. the structural, high strength types, notably epoxy type resins.
In the past steel surfaces were prepared for adhesive bonding by wire brushing, abrasive blasting and cleaning with chemicals and organic solvents. The past methods were adequate but in many cases had significant shortcomings. We have recently found that highly satisfactory, strong, durable bonds on steel can be obtained by etching the steel surface with a mixture of phosphoric acid and ethanol prior to bonding with an adhesive. However, the use of such etching baths is impractical for industrial applications primarily in view of their high alcohol content, since the ethanol vapors over the etching tank are highly flammable and somewhat toxic. Accordingly, there is a need for an etching method and an etching bath composition, which avoids such disadvantages but provides similar strong, durable bonds on steel.